The intake of food and its conversion in the body is an essential part of life for all living creatures. Therefore, deviations in the intake and conversion of food generally lead to problems and also illness. The changes in the lifestyle and nutrition of humans, particularly in industrialised countries, have promoted morbid overweight (also known as corpulence or obesity) in recent decades. In affected people, obesity leads directly to restricted mobility and a reduction in the quality of life. There is the additional factor that obesity often leads to other diseases such as, for example, diabetes, dyslipidaemia, high blood pressure, arteriosclerosis and coronary heart disease. Moreover, high body weight alone puts an increased strain on the support and mobility apparatus, which can lead to chronic pain and diseases such as arthritis or osteoarthritis. Thus, obesity is a serious health problem for society.
The term obesity means an excess of adipose tissue in the body. In this connection, obesity is fundamentally to be seen as the increased level of fatness which leads to a health risk. There is no sharp distinction between normal individuals and those suffering from obesity, but the health risk accompanying obesity is presumed to rise continuously as the level of fatness increases.
Apart from physical activity and a change in nutrition, there is currently no convincing treatment option for effectively reducing body weight. However, as obesity is a major risk factor in the development of serious and even life-threatening diseases, it is all the more important to have access to pharmaceutical active substances for the prevention and/or treatment of obesity. One approach which has been proposed is the therapeutic use of MCH antagonists.
Investigations into the function of MCH in animal models have provided good indications for a role of the peptide in regulating the energy balance, i.e., changing metabolic activity and food intake. For example, after intraventricular administration of MCH in rats, food intake was increased compared with control animals. Additionally, transgenic rats which produce more MCH than control animals, when given a high-fat diet, responded by gaining significantly more weight than animals without an experimentally altered MCH level. It was also found that there is a positive correlation between phases of increased desire for food and the quantity of MCH mRNA in the hypothalamus of rats. On the other hand, experiments with MCH knock-out mice are particularly important in showing the function of MCH. Loss of the neuropeptide results in lean animals with a reduced fat mass, which take in significantly less food than control animals.
The anorectic effects of MCH are presumably mediated in rodents through the Gαs-coupled MCH-1 receptor (MCH-1R), as, unlike primates, ferrets and dogs, no second MCH receptor subtype has hitherto been found in rodents. After losing the MCH-1R, knock-out mice have a lower fat mass, an increased energy conversion and, when fed on a high fat diet, do not put on weight, compared with control animals. Several small-molecule MCH-1R antagonists have been described in the literature which consistently show decreased food intake and body weight reduction in animal models of diet-induced obesity [1].
In addition to its anorectic effect, MCH-1R antagonists also achieve additional anxiolytic and antidepressant effects in behavioural experiments [1]. Thus, there are clear indications that the MCH-MCH-1R system is involved not only in regulating the energy balance but also in affectivity.